Advanced Cell Test (ACT) Study
Cognition and Imperial College London collaborated on this Innovate UK funded project that started in October 2020 and is still gathering data to compare traditional cycling methods with advanced test techniques to predict cell life and the critical rollover point where cell capacity rapidly decreases.
A single commercially available 18650 Li-ion cell was selected for study and a test plan generated. Due to our wide range of cycling channels available, Cognition carried out life cycling experiments, while Imperial College carried out extensive advanced tests. All cell cycling was performed in prototype CellPods, providing a surface cooled environment for cell cycling, with Cognition fitting out 96 channels with them and Imperial 24, see Figure 1.
Figure 1: Prototype CellPods running ACT tests.
Life Cycle Tests
Life cycle tests under a range of conditions were studied to provide a detailed dataset for reference cell lifetime. The following parameters were varied;
4 different power rates were chosen, to be varied on both charge and discharge, for a total of 16 different power profiles to be tested.
4 different temperatures were selected to be tested.
3 repeats were required for each point of the test matrix for the 64 total unique test conditions.
Consequently, 192 cells in total underwent life cycle testing.
Imperial College and Cognition identified a number of advanced tests that could help predict cell life and developed suitable test plans. These included: High Precision Coulometry (HPC) where a subset of the life cycle tests were performed to accurately measure coulombic efficiency, see Figure 2; and Differential Thermal Analysis (DTA) on cells periodically as they cycled. Example results are shown in Figure 3. Other methods, such as Electrochemical Impedance Spectroscopy (EIS), and Incremental Capacity Analysis (ICA) were applied to the cells.
Figure 2. High Precision Coulometry (HPC) results for Coulombic Efficiency at different temperatures.
Figure 3: Differential Thermal Analysis results for a cell measured every 100 life cycles.
Analysis & Prediction
Cognition and Imperial collated all of the data and analysed it on a weekly basis. This was only possible with automated MATLAB analysis for the life cycle data due to the quantity being gathered. The total cycling data gathered to date exceeds 600,000 hours of testing and is still growing as testing has not yet completed, it is expected that the final dataset will exceed 1,000,000 hours.
The initial conclusions of the study, prior to completion of the test, are as follows:
Unsurprisingly, cycling a cell cold leads to more rapid degradation than cycling at a high temperature, and higher power rates also accelerates degradation (See Figure 4).
HPC is a useful tool to help determine the ideal temperature and power rates for a cell to achieve maximum possible lifetime
DTA is an effective method to non-invasively measure electrolyte degradation, with low temperature cycling seeing more rapid changes than high temperature cycling, aligning with the trends in life cycle degradation seen.
An academic paper is planned with Imperial College London, examining the use of the DTA testing technique for life cycle prediction in detail.
Figure 4: Selected cycle life data gather during the ACT study for cells cycled under identical conditions at 3 temperatures.